1. Field of the Invention
The present invention relates to a lens system, and in particular, to a lens system in which focus adjustment of a taking lens can be switched between manual focus adjustment and auto-focus adjustment.
2. Description of the Related Art
Focus adjustment of a camera lens includes auto-focus (AF) with which focusing on a subject is executed automatically and manual focus (MF) with which an operator operates an operation member to execute focusing on a subject. Various systems are known as the AF. In particular, AF of a contrast system is generally used in a video camera for consumer use, a television camera for broadcast, or the like. For example, a high-frequency component is extracted from a video signal obtained by an image pickup element and the high-frequency component is integrated within an AF object area (focus area), whereby an integrated value is found. This integrated value indicates a sharpness (high and low of a contrast) of an image. A camera lens is focused on the subject within the focus area by executing focus adjustment such that the integrated value becomes largest (maximal). The integrated value is referred to as a focus evaluation value in this specification.
Usually, the AF and MF are switched by a predetermined mode changeover switch. In general, in the video camera for consumer use, an AF mode or an MF mode is selected by the mode changeover switch, whereby focus adjustment can be executed with the AF or the MF.
On the other hand, in the television camera for broadcast, it is impossible to execute focus adjustment only with the AF entirely. Japanese Patent Application Publication No. 8-29665 discloses a focus adjustment method which makes it possible to execute focus adjustment in accordance with an operation of the MF when an operator executes the operation of the MF even if a camera is in the AF mode. According to this method, there is an advantage that, for example, in the case where a subject focused on with the AF is not a subject which the operator desires, focus can be immediately corrected according to the operation of the MF by the operator.
In Japanese Patent Application Publication No. 8-29665, although processing of the AF is resumed after the operation of the MF is stopped, at this point, there are two cases that are taken into account: the case where focus is prohibited from moving with the AF even if a focus evaluation value has not reached a peak point unless a state of a subject changes; and the case where focus is moved to the peak point by the AF when the focus evaluation value is not the peak point even if a state of the subject does not change.
The former case is particularly effective in the case where a camera lens is intentionally made unfocused by the MF, the case where a camera lens is focused on a subject, which is not judged as focused by the AF, by the MF, and the like.
On the other hand, the latter case is particularly effective, in the case where there are a plurality of subjects on which a camera lens can be possibly focused on at different distances from the camera (i.e., in the case of a picture in which a focus evaluation value indicates the peak value in a plurality of focus positions), when an operator desires to change a subject on which the camera lens is focused by the AF. For example, in the case where the camera lens is not focused on a target subject by the first AF, the camera lens is focused on the target subject by the MF. In this case, even if focusing is not executed accurately, focus moves such that a focus evaluation point becomes the peak point by processing of the AF when an operation of the MF stops and the camera lens is accurately focused on the target subject. In the above-described AF of the contrast system, a so-called mountain-climbing system is adopted with which, in general, focus is moved in a direction in which a focus evaluation value increases to find a peak point of the focus evaluation value, and the focus is stopped in a position of the peak point. Thus, if the focus is moved to the vicinity of the peak point of a mountain-shape distribution of focus evaluation values formed by a target subject, the focus moves to the peak point of the mountain-shape distribution with the AF after the MF and the camera lens is focused on the subject.
Conventionally, focus adjustment is often executed by the MF in the television camera for broadcast. Thus, it has been proposed that a television camera which is premised on focus adjustment in the MF mode, processing of the AF is automatically started after an operation of the MF stops and the television camera is focused only once (so-called one-shot AF) and, thereafter, it returns to the MF (e.g., see Japanese Patent Application Publication No. 62-187829).
In the focus adjustment method as proposed in Japanese Patent Application Publication No. 8-29665, in the case where the conventional AF of the contrast system is adopted, there is a problem in that it is hard to show the effect as described above. That is, in the case where focus is stopped unless a state of a subject changes after an operation of the MF stops, whether or not the state of the subject has changed is judged according to a change in a focus evaluation value. However, if there is a change in illumination or flicker, since a focus evaluation value changes even if a state of a subject changes, there is an inconvenience in that focus moves due to the AF and cannot be kept stopped in a position to which the focus has moved by the operation of the MF. In the case where, after an operation of the MF stops, focus is moved to a peak point by the AF when a focus evaluation value is not the peak point, it is necessary to judge whether or not the focus evaluation value is the peak point and to judge an increasing direction of the focus evaluation value if it is judged that the focus evaluation value is not the peak point. Thus, for example, it is necessary to execute an operation, so-called wobbling, for determining a change in a focus evaluation value by moving focus back and forth, and there is a disadvantage that switching from the MF to the AF becomes unnatural. There is also a disadvantage that an unexpected situation tends to occur, for example, a focus evaluation value fluctuates due to a change in illumination or flicker even if a state of a subject does not change and a camera lens is not focused on the subject.
In the method which was proposed in the past concerning focus adjustment using both the AF and the MF, there is a problem in that it is inconvenient for an operator who wishes to execute focus adjustment mainly using the MF and using the AF supportively. That is, troublesome labor is required in that the operator usually sets the MF mode to execute focus adjustment with the MF, and for example, in the case where the operator wishes to use the AF only for a short time, after switching to the AF mode once to execute the AF, it is necessary to switch the AF mode to the MF mode again. In the case of Japanese Patent Application Publication No. 62-187829, the AF can be used supportively without any labor. However, in this case, it is an object to execute focusing of a higher accuracy, which cannot be confirmed by a viewfinder, with the AF as opposed to focusing with the MF of the operator. In this form, continuous AF (AF continuously actuating the AF as opposed to the one-shot AF is referred to as continuous AF) cannot be executed. Final focus adjustment cannot be executed with the MF either in the combination of the AF and the MF. For example, this form cannot cope with the case where the operator wishes to mainly use the MF and use the continuous AF only for a short time, the case where the operator wishes to make a camera unfocused intentionally during the continuous AF, the case where focus is corrected with the MF if a moving subject is tracked by the continuous AF and a camera lens is unfocused largely, and the like.